HE heat transfer coefficient inquiry!

[gbratis]

Hi everybody,
This is my first thread in this mechanical engineering division so my question is simple.Often we are asked to size shell-u tube heat exchangers and as far I know the most critical parameter is defining the overall heat transfer coefficient. Different approaches-values, result to alternative designs, so my inquiry has to do with a reliable source from which these values can be obtained with a safety margin. For example in natural gas-water shell(water) tube (natural gas) HE we use U=120 Btu/hr/ft2/deg.F. Is this value conservative?. Can we go with a higher value.?
Thanks in advance.

 

[quark]

This one is a reliable source.

http://www.processassociates.com/process/heat/uvalues1.htm

Better option is to calculate U values from individual heat transfer coefficients of shell side and tube side and then deriving overall heat transfer coefficient. You can refer either Holman or Incropera & Dewitt.

 

[25362]

Kindly consider the OHTC is always lower than the lowest individual HTC, and fouling factors should be selected on basis of accumulated experience.

For ballpark work your U coefficient doesn't seem to be conservative, it may even be too optimistic. Anyway, I suggest you follow quark's advice.

OHTC = U = overall heat transfer coefficient

 

[davefitz]

In addition to determining the OTC, you also need to make a corection due to the HX configuration- ie, number of passes, counterflow vs parrellel flow, etc. The standard estimate of using LMTD is not accurate unles you have at least 5 passes or one of the fluids has an effectively infinite Cp.

Se compact heat exchanger theory , e- NTU method, etc.

 

[srfish]

What is the operating pressure on the natural gas ? The pressure is an indication of the value of the natural gas heat transfer coefficient. This coefficient will vary approximately with the square root of the operating pressure.

 

[abeltio]

start with Donald Q. Kern, Process Heat Transfer

saludos.
a.

 

[WebWizz]

Another good resource:

http://www.engineeringpage.com

Provides both experience values as calculation routines.

Regards